Rotary atomizers are a type of liquid spray coating device which includes an atomizer head rotatable at high speed (typically 10,000-45,000 revolutions per minute) by an air turbine motor to apply liquid coating material, such as paint, in atomized form onto the surface of a workpiece. The atomizer head is usually in the form of a disc or cup which includes an interior wall that defines a cavity and terminates in an atomizing edge. Liquid coating material delivered to the interior of the cup flows outwardly under centrifugal force along the interior wall of the cup and is expelled radially outward from the peripheral edge of the cup to form a spray pattern of atomized droplets of coating material. To improve the transfer efficiency of the coating process, an electrostatic charge is imparted to the coating material so that the pattern of atomized coating material is attracted to an electrically grounded workpiece.
An example of an electrostatically charged rotary atomizer is disclosed in commonly assigned U.S. Pat. No. 4,887,770 ('770) to Wacker et al., which is expressly incorporated herein in its entirety by reference.
Prior to the '770 patent, one of the hazards associated with the use of the conductive atomizing cup was the possibility of operator shock or ignition of combustible coatings because of the high voltage at which the cups were maintained. For example, as disclosed in U.S. Pat. No. 4,369,924, a charge is transferred through a turbine shaft from a power supply to the rotary atomizer cup. Since, both the cup and the entire rotary atomizing housing were metal and were charged to a high voltage, there is a significant safety hazard since the atomizer carries sufficient charge to severely shock an operator. Therefore, protective fences and interlocks have to be installed around the atomizer.
The '770 patent, listed before, discloses a low capacitance, rotary atomizer which, while electro-statically charging the coating paint at the rotary atomizer cup, does not store sufficient charge to present a shock hazard and therefore does not have to be protected by fences and safety interlocks. To charge the atomizer in the '770 patent, external electrode probes (462) direct the charge into the cup (20). Still, under certain strict testing conditions, a charge on the equipment could still cause some safety concerns.
Another problem associated with prior art rotary atomizers is that the rotary atomizer cups have not been easy to disassemble and clean. For example, in U.S. Pat. No. 4,838,487, a deflecting member (28) is held in place against atomizing bell (10) by spacers (36). However, in operation, dried paint can collect on the front surface (30) of the deflector member. Then, the flow of paint across the front surface with the dried paint has a tendency to form an irregular coating on the part being sprayed.
Still another problem associated with the prior art electrostatic spray guns was related to the isolation of the spray gun with the liquid supply. One solution, as described in U.S. Pat. No. 4,139,155 to Hastings was to provide a spiral conduit in the flow passage.